1. Field of the Invention
The present invention relates to a method of forming a polycrystalline silicon layer for a switching element, for example, a thin film transistor (TFT).
2. Description of Related Art
A thin film transistor (TFT) includes an insulating layer, a passivation film, electrode layers and a semiconductor layer. The insulating layer is beneficially comprised of SiNx, SiO2, Al2O3 or TaOx. The passivation film is beneficially comprised of a transparent organic insulating material or of a transparent inorganic insulating material. The electrode layers include a gate electrode, a source electrode, and a drain electrode, and are beneficially comprised of a conductive metal such as Al, Cr or Mo. The semiconductor layer acts as a channel region along which charges flow. The semiconductor layer is usually comprised of amorphous silicon or polycrystalline silicon.
A process of forming the semiconductor layer using amorphous silicon can be performed at a low temperature of about 350° C. and is relatively simple. However, since a field effect mobility of an amorphous silicon layer can be as low as 2 cm2/V·sec, the switching characteristics of the TFT are not particularly good.
A polycrystalline silicon layer has a much better speed of response than an amorphous silicon layer. A polycrystalline silicon layer can have a field effect mobility between 20 cm2/V·sec to about 550 cm2/V·sec. As the switching speed of a TFT depends on the field effect mobility, the switching speed of a TFT having a polycrystalline silicon layer is about 100 times as fast as that of a similar TFT having an amorphous silicon layer. The higher field effect mobility is a result of the better grains in a polycrystalline silicon layer as compared to those of an amorphous silicon layer.
Methods of forming polycrystalline silicon layers include an excimer laser annealing technique, a solid phase crystallization (SPC) technique, and a metal-induced crystallization (MIC) technique.
Excimer laser annealing is usually performed at a low temperature and, thus a low-cost glass substrate can be used. A TFT manufactured using the excimer laser annealing technique usually has a field effect mobility of more than 100 cm2/V·sec, and thus has excellent operating characteristics.
The solid phase crystallization technique is one in which amorphous silicon is crystallized at a high temperature, usually over 600° C. Since crystallization is performed at a solid phase, a grain can have defects such as a micro-twin, a dislocation, and the like. Grains obtained using solid phase crystallization are usually of a low grade. In order to compensate for this, a thermal oxidation film formed at about 1000° C. is usually used as a gate insulating layer. Therefore, a high-cost material, such as quartz, is required for the substrate. This presents a problem in that production costs are high.
In the metal-induced crystallization technique, crystallization is performed by depositing a metal layer on an amorphous silicon layer which is then heat treated. The metal layer lowers the enthalpy of the amorphous silicon layer. As a result, crystallization is possible at a low temperature of about 500° C. However, the resulting surface state and electrical characteristics are not particularly good, and the resulting grains can have many defects.
A polycrystalline silicon layer manufactured using the techniques described above can obtain grains from silicon seeds formed while the silicon, in a liquid state, cools. If a silicon grain grows laterally, large-sized grains can be obtained. If the distance between adjacent silicon seeds is greater than the maximum silicon growth distance, a silicon grain grows laterally with the silicon seed at the center of the grain. After the silicon grain grows to its maximum length, nuclei generated by super-cooling create many small-sized grains. However, if the distance between adjacent silicon seeds is less than the maximum silicon growth distance, large sized grains that meet at grain boundaries are formed. To obtain a high-quality TFT, large-sized grains should be uniformly arranged while forming grain boundaries.
FIGS. 1A to 1C are plan views illustrating a crystallization process of a polycrystalline silicon layer. The distance between adjacent silicon seeds 11 is less than the maximum grain growth distance. It is desirable that the silicon seeds 11 are uniformly distributed. The silicon grains 13 grow laterally, centering on the silicon seed 11, and complete their growth after forming grain boundaries 15.
Hereinafter, a crystallization process using the excimer laser annealing technique according to a conventional art is explained in more detail.
FIG. 2 is a perspective view illustrating a configuration of polycrystalline silicon crystallization equipment using the excimer laser annealing technique. The equipment includes a laser beam device (not shown), a mask 33, and a projection lens 35. The projection lens 35 is arranged over a substrate 31, and the mask 33 is aligned with the projection lens 35. When a laser beam 37 is projected from the laser beam device toward the mask 33, the laser beam 37 becomes incident on the mask. The laser beam incident to the mask 33 passes through the projection lens 35 and is concentrated on the substrate 31, which has an amorphous silicon layer. Polycrystallization of the amorphous silicon layer is performed according to the mask pattern.
At this point, a growth of the polycrystalline grain is controlled by the shape and energy density of the laser beam, and by the temperature and cooling rate of the substrate. During crystallization, a silicon grain has three regions: a low energy density region; an intermediate energy density region; and a high energy density region. The low energy density region is a partially melted region. That is, the lower energy density region is one in which the lower portion of the silicon layer is not melted, and thus the silicon melting depth is less than the thickness of the silicon layer. The resulting grain diameter is less than the thickness of the silicon layer because seeds on the lower portion of the silicon layer tend to grow vertically.
The intermediate energy density region is an almost completely melted region. That is, the intermediate energy density is one which only some of seeds on the lower portion of the silicon layer are not completely melted. Except for those seeds, almost all of the silicon layer is completely melted. This region is a region in which lateral growth, centered on the seeds, is possible.
The high energy density region is one in which even the lowest portion of the silicon layer is completely melted.
A crystallization method using the polycrystalline silicon crystallization equipment of FIG. 2 is as follows. The laser beam 37 is uniformed by predetermined means. Thereafter, the laser beam that will be formed on the substrate 31 is determined by the mask 33. A laser beam having a width of tens of μm is formed through the projection lens 35. The substrate 31 is arranged on a stage that moves slowly, usually at a speed of less than 1 μm/pulse, so that crystallization is performed by the laser beam. The mask 33 has divided stripe shape regions “A”, “B”, and “C”.
FIG. 3 is a plan view illustrating a mechanism for laser crystallization of amorphous silicon. In a first crystallization step, shown in element 45, lateral growth occurs by moving the substrate 31. In the second crystallization step, also shown in element 45, a grain boundary 41 formed in the first step moves and forms a new grain boundary 41a. Preferably, a high energy density that completely melts the silicon is used, and the width of the laser beam is less than twice the maximum lateral growth distance.
The step and melt process continues. After an n-th crystallization step, grains 43 of the polycrystalline silicon that were crystallized by the lateral growth are large, and the grain boundary 41n is finally determined.
FIG. 4 is an enlarged view illustrating a portion D of FIG. 3. As shown in FIG. 4, the polycrystalline silicon layer has a protruding portion 45 that protrudes upwardly. The protruding portion is formed on the grain boundary due to growth of adjacent grains. This result comes about because solid silicon is greater in volume than liquid silicon, and the silicon layer melted last is cooled at the grain boundary region, increasing its volume. The protruding portion 45 has a height of about 300 Å.
Further, when the silicon layer is crystallized using the conventional crystallization described above, as shown in FIG. 5, defects 51, each referred to as a low angle defect, may exist on the surface of the layer. This result comes about because the heat energy contained in the silicon layer is suddenly exhausted via the substrate below the silicon layer when the laser beam is blocked.
The defects on the surface of the silicon layer result from sudden cooling, leading to abnormal grain growth.
A polycrystalline silicon layer manufactured by the above-described method is usually patterned to form a channel in subsequent processes. Then, an insulating layer is formed on the polycrystalline silicon layer. In other words, since the insulating layer is formed on a non-flat surface due to grain defects and the grain boundary protruding upwardly, a trap level may occur due to a mismatch between the polycrystalline silicon layer and the insulating layer. The result is that the field effect mobility of charges that flow along a surface of the polycrystalline silicon layer is significantly lowered, leading to low reliability.